Flexographic printing plates are prepared using photopolymerizable recording materials on which the printing surface is produced by exposing imagewise a light-sensitive layer and removing the unexposed parts of the layer. Solid photopolymerizable materials can be differentiated from those in which the imagewise exposure is made on liquid systems. Following imagewise exposure, the photopolymerizable recording materials can then be developed in an aqueous or organic solvent system.
Examples of solid, solvent-developable recording materials are disclosed in DE-C3 22 15 090; U.S. Pat. Nos. 4,197,130; 4,320,188, and 4,430,417. Such materials usually consist of a support, an optional adhesive or other base coating, a solid, photopolymerizable layer and a cover element, consisting of a flexible polymer film that can be removed by the solvent developer, and a separable cover layer. The solid, photopolymerizable layer contains (1) at least one polymeric binder, (2) at least one ethylenically unsaturated compound that is addition-polymerizable by actinic light, and (3) a photoinitiator or photoinitiator system.
Printing plates can be prepared without a separate support. For example, DE-B 17 72 662 discloses the preparation of a printing plate with an integrated support from thick layers of photopolymerizable mixtures, particularly liquid mixtures. The plate is exposed overall on the side opposite the side that was imagewise exposed. The overall exposure can be made before, after, or simultaneously with the imagewise main exposure.
DE-C2 25 42 815 describes a process in which small projections are produced in the image-free areas by a backside exposure made through a screen before, after, or simultaneously with the imagewise exposure to prevent ink pickup in these areas.
DE-A 20 61 287 discloses a process for preparing gravure printing plates, in which one side is imagewise exposed and the other side is exposed through a screen. The printing surface of these gravure printing plates is the side that is exposed through the master screen.
U.S. Pat. No. 3,408,191 describes a process for the simultaneous preparation of positive and negative images by a peel-apart process. A photopolymerizable layer located between two supports is exposed imagewise through one support and exposed overall through the other support. The layered material is pulled apart and the positive image remains on the support that was exposed overall.
In preparing flexographic printing plates, the conventional practice has been to expose overall through the support before exposing imagewise from the cover layer side. See E Schulz, "Flexodruck von A bis Z", Polygraph, Frankfurt am Main, 1987, pages 92-94. The preliminary exposure step anchors the printing areas firmly to the support, builds a plate base, the so-called "floor", and increases sensitivity for the imagewise main exposure, as described in DE-C 12 14 085.
A disadvantage to using this conventional practice is that exposure latitude is clearly decreased. It is very difficult to achieve simultaneously good anchoring of the smallest dots in the highlights and adequate shadow relief depths of at least 100 .mu.m in fine negative image elements for high resolution in printing. A very long exposure time is always necessary for complete polymerization of small dots. Consequently, the fine negative lines are frequently filled in by polymerization and the printing plate cannot be used.
Attempts to shorten or completely dispense with backside exposure in order to increase exposure latitude and to avoid polymerizing together the fine negative lines have shown that the smallest dots cannot be polymerized sufficiently. In fact, the dots can be deformed easily or even washed off in the development process. Missing smaller dots or merged negative lines are unacceptable for high quality printing. Very good image resolution in the highlights as well as shadows and accurate tonal value reproduction are very much required. Thus, a firm floor and consequently, uniform relief depth cannot be attained without backside exposure because these are affected by the intensity of the washoff process.
To fulfill these requirements, the preliminary exposure has been performed in practice until now through the support. See for example, U.S. Pat. No. 4,320,188 and EP-B1 00 84 851). In addition, attention has been concentrated on adhering accurately to the optimum exposure time that must be predetermined specifically for each plate type. Nevertheless, the production of useless printing plates cannot be avoided by this approach. Consequently, the preferred production of printing plates having improved printing quality employs methods that use a custom plate formulation for each special problem. Naturally, this increases development costs and moreover, requires maintaining a supply of many different photopolymerizable printing plates.
Various versions of exposure processes have also been proposed, but all are costly and require exacting operations. Thus, for many years, a technique was practiced in which fine negative lines are masked for part of the exposure time.
EP-A2 02 95 818 describes a process in which the photopolymerizable printing plate is exposed imagewise from the cover layer side through a negative and on the support side, through the corresponding positive. Thus, the floor is built up selectively in image-free areas. The positive and negative exposure can be performed simultaneously as well as in any desired sequence. To produce a printing plate by this process, two exposure masks, one positive and one negative, are required. In addition, exact registration positioning of the negative or positive is required.
EP-A3 01 69 294 discloses a process in which a liquid system is exposed imagewise from one side through a negative to form the printing surface and is exposed similarly imagewise on the other side through a second negative. However, the second negative has somewhat larger transparent areas than the first negative, so that the printing reliefs are anchored on a wider floor. The integrated support is formed, as is usual in liquid systems, by an overall backside exposure.
EP-A3 02 31 902 describes as process for preparing a continuously coated printing cylinder by heat-sealing the abutting edges of a printing plate mounted on the printing cylinder. In this case, the preliminary backside exposure is performed through a screen, because, on one hand, printing plates pre-exposed overall cannot then be heat-sealed, while, on the other hand, the preliminary exposure cannot be omitted for the above-stated reasons. In this manner, the layer remains heat-sealable.
Most currently known processes for improving the printing quality of flexographic printing plates are complicated, time-consuming, costly and frequently useful only for special problems.
Accordingly, the object of the present invention is to provide a process for preparing flexographic printing plates with which it is possible to obtain fast, simple and high quality printing with the smallest dots well anchored, fine negative lines completely defined, and very high image resolution on printing. In addition, dot growth and thereby, tonal value growth should be minimized and exposure latitude widened. Other properties of printing plates, such as, for example, their hardness or resiliency should not be affected adversely.